(inkstain) – A warming climate is already reducing the flow in the Colorado River, and the future risk is large, with a worst case of the river’s flow being cut in half by the end of the century, according to a new study from a pair of the region’s leading researchers. While precipitation declines since the turn of the century have been modest, Brad Udall and Jonathan Overpeck found, a little less rain and snow have translated to a lot less water in the river […]

While we have seen projections of the future impact of climate change on the Colorado for more than two decades, this study is important because it is one of the first to argue empirically that the change is already underway. (See also Woodhouse and colleagues last year, which I wrote about here.) [more]

ABSTRACT: Between 2000 and 2014, annual Colorado River flows averaged 19% below the 1906-1999 average, the worst 15-year drought on record. At least one-sixth to one-half (average at one-third) of this loss is due to unprecedented temperatures (0.9°C above the 1906-99 average), confirming model-based analysis that continued warming will likely further reduce flows. Whereas it is virtually certain that warming will continue with additional emissions of greenhouse gases to the atmosphere, there has been no observed trend towards greater precipitation in the Colorado Basin, nor are climate models in agreement that there should be a trend. Moreover, there is a significant risk of decadal and multidecadal drought in the coming century, indicating that any increase in mean precipitation will likely be offset during periods of prolonged drought. Recently published estimates of Colorado River flow sensitivity to temperature combined with a large number of recent climate model-based temperature projections indicate that continued business-as-usual warming will drive temperature-induced declines in river flow, conservatively -20% by mid-century and -35% by end–century, with support for losses exceeding -30% at mid-century and -55% at end-century. Precipitation increases may moderate these declines somewhat, but to date no such increases are evident and there is no model agreement on future precipitation changes. These results, combined with the increasing likelihood of prolonged drought in the river basin, suggest that future climate change impacts on the Colorado River flows will be much more serious than currently assumed, especially if substantial reductions in greenhouse gas emissions do not occur.